It is often desirable in nuclear reactor systems to produce large or substantial neutron fluxes and high fast-to-thermal neutron ratios, either in order to test and analyze the performance of new reactor fuels and materials in such environments or to otherwise operate nuclear reactor systems under “fast-flux” radiation conditions. One method for testing materials under fast-flux radiation conditions is to place the materials in a reactor and expose them to fast neutrons. More particularly, it is often desirable to conduct such testing under conditions of high fast-to-thermal neutron ratios (e.g., ratios of about 15 or greater) as well as at large or substantial neutron flux intensities (e.g., 1×1015 n/cm2·s or greater). Unfortunately, these neutron energy and flux conditions place conflicting technical requirements on the configuration and operational parameters of the reactor system.
For example, the requirement for a high fast-to-thermal neutron ratio (e.g., in excess of 15) typically requires the absence of significant neutron thermalizer between the neutron source and the target material. In addition, neutron “filters” may need to be provided to remove or filter low energy or thermal neutrons from the neutron flux. The requirement for a high neutron flux intensity (e.g., in excess of 1015 n/cm2·s) typically results in substantial heating of the target material being studied, and may involve the use of booster fuels that further add to the heat load. In addition, such high flux intensities will result in additional heating of the neutron filter, e.g., from the (n,γ) absorption reactions, which additional heat must also be somehow removed from the apparatus.
While numerous types of cooling systems are known and may be used for this purpose, they too, present conflicting design requirements. For example, while gas cooling systems are known and may be used, they must be operated at substantial pressures and flow rates in order to remove the excessive heat generated as a result of the high neutron flux intensities. Past experience has indicated that gas cooling systems are expensive and difficult to operate in such regimes. While molten metal and molten salt cooling systems are also known and may be used, they are not without their problems. For example, the use of liquid metals and salts may present safety concerns if they are reactive with the primary coolant being used in the test reactor. Moreover, such molten coolants (e.g., either metals or salts), must be maintained in the molten state in order to avoid structural damage to the system. Water cooled systems are also known and could be used. However, water is a highly effective neutron thermalizer, and thus serves to lower the fast-to-thermal neutron flux ratio.
Consequently, the task of designing a reactor system suitable for exposing materials to high fast-to-thermal neutron flux ratios and at high flux intensities is by no means trivial and presents a number of conflicting technical and economic requirements that must be resolved in order to arrive at a successful system.